Methods and apparatus of providing transmit diversity in a multiple access wireless communication system

ABSTRACT

Methods and apparatus for providing channel diversity to wireless terminals (WTs) in a manner that reduces the latency between the time a WT encounters satisfactory channel conditions are described. A plurality of communications channels with different physical characteristics are maintained in a cell by a base station (BS). Each WT monitors multiple channels and maintains multiple channel estimates at the same time so that rapid switching between channels is possible. Channel quality information is conveyed from each WT to the BS. The WT or BS selects a channel based on the measured channel quality. By supporting multiple channels and by introducing periodic variations into the channels in various embodiments, the time before a WT encounters a channel with good or acceptable channel conditions is minimized even if the WT does not change location. Multiple antennas are used at the BS to support numerous channels simultaneously, e.g., by controlling antenna patterns.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 10/763,944, filed Jan. 23, 2004, now allowed, entitled METHODSAND APPARATUS OF PROVIDING TRASMIT DIVERSITY IN A MULTIPLE ACCESSWIRELESS COMMUNICATION SYSTEM, which claims the benefit of U.S.Provisional Patent Application Ser. No. 60/442,008, filed Jan. 23, 2003,entitled “METHODS AND APPARATUS OF PROVIDING TRANSMIT DIVERSITY IN AMULTIPLE ACCESS WIRELESS COMMUNICATION SYSTEM” and U.S. ProvisionalPatent Application Ser. No. 60/509,741, filed Oct. 8, 2003 entitled“METHODS AND APPARATUS OF PROVIDING TRANSMIT DIVERSITY IN A MULTIPLEACCESS WIRELESS COMMUNICATION SYSTEM” each of which is hereby expresslyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates to communications systems and, moreparticularly, to methods and apparatus for providing transmit diversityin a multiple access cellular communications network.

BACKGROUND

In a wireless communication system, a base station, situated at a fixedlocation, communicates with a plurality of wireless terminals, e.g.,mobile nodes that may move throughout its cell. A given base station,with a single fixed antenna may have a fixed antenna pattern. Consider asingle base station; its antenna pattern will support variable levels ofchannel quality between the base station and mobile nodes, depending onthe mobile node's location with respect to the antenna pattern. Nowconsider that an adjacent base station, with its own antenna pattern,may be creating different levels of interference at different locations.The channel quality between the base station and a mobile node will varyas the mobile node moves to different locations within the cell. Themobile node may experience fading resulting in a degradations or loss ofcommunication. Certain areas within the cell may be considered deadzones where the channel quality is too poor to establish communications.Methods and apparatus are needed that reduce fading and dead zoneswithin cells.

In a system, with many mobile nodes, there will typically be a largediversity among the population of users, e.g., for any given antennapattern there will be some users with good channel condition, some userswith poor channel conditions, and other users with varying levels ofchannel conditions. At any given instant of time each mobile nodeexperiences quasi-static channel conditions. Pilot signals may bebroadcast to the mobile nodes; each mobile node's channel quality may bemeasured and reported back to the base station. Therefore, a basestation could schedule mobile nodes with good channel quality, andhold-off scheduling mobile nodes with poor channel quality. When such amethod is used in a strict manner, a mobile node, with poor channelquality, might have to move to a location with acceptable channelquality in order to be scheduled by the base station.

In another approach, the base station could periodically readjust itsantenna pattern, again send pilot signals, wait for channel qualityreports from the mobile node and schedule those mobile nodes with goodchannel quality. This second approach may lead to a long delay for amobile node situated in a location of poor channel quality before thebase station antenna pattern is adjusted to an acceptable level. Inaddition, this second approach favors one set of mobile nodes at theexpense of another set of mobile nodes. The scheduling delays involvedwith either of these approaches may be unacceptable for certain types ofdelay-sensitive traffic such as voice. In some cases, if the traffic ofthe user has stringent delay constraints, the base station may, beforced to schedule a user even when channel conditions are not favorableresulting in a poor quality of service. Thus, for real time applicationssuch as voice, it is often important to minimize the time period betweentransmission to a wireless terminal.

In cases where a channel's conditions are varied, practical constraintslimit the rate at which the conditions in a particular channel may bevaried without negatively impacting communications system performance.From a wireless terminal's perspective, rapid changes in acommunications channel are difficult to track. Furthermore, rapidchanges often result in a channel estimate used to decode a receivedsignal being inaccurate since the channel conditions may have changedsignificantly since the channel measurements upon which the channelestimate is based were made. The use of feedback loops between a basestation and a wireless terminal for power control and other purposeslimits the rate at which communications channels can be varied sincevarying channel conditions at a rate faster than the rate at whichchannel condition information is measured by a wireless terminal and fedback to the base station can lead to the base station having largelyinaccurate channel condition information.

In view of the above discussion, is should be appreciated that there isa need for improved methods and apparatus for supporting communicationto multiple wireless terminals in a cell which may be distributedthroughout the cell. Improved methods for providing a mobile withsuitable channel conditions for receiving information from a basestation are needed. From a scheduling perspective, it would bebeneficial if the time interval between periods where a wirelessterminal in a cell encounters good channel conditions could be minimizedso that the wireless terminal need not have a long delay beforeencountering suitable transmission conditions. If intentional channelvariations are used, it is desirable that the rate at which variationsare introduced into a channel be slower than the rate at which channelmeasurements are made by the wireless terminals and/or the rate at whichchannel condition information is feed back to the base station. It wouldbe desirable if at least some new methods address the problem of therelative duration of a mobile node's quasi-static channel conditionrelative to an acceptable scheduling latency. Methods and apparatus thataddress ways to mitigate interference effects from adjacent cells wouldalso be beneficial. Methods that exploit the user diversity of thesystem, rather than be constrained by it, would also be beneficial. Suchimproved methods could increase user satisfaction, increase quality ofservice, increase efficiency, and/or increase throughput.

SUMMARY

The present invention is directed to methods and apparatus for improvingreducing scheduling latency in a communication system. In accordancewith the present invention, multiple communications channels aremaintained by a basestation with different physical characteristics andeach of the communications channels occupies a portion of the availablecommunications resource. The physical partition of the availablecommunications resource into multiple parallel communication channelswith different physical characteristics can be done in a variety of wayssuch as in frequency, in time, or in code, or some combination of these.In some embodiment, the communications channels are orthogonal to eachother.

Each wireless terminal measures the channel conditions on differentcommunications channels. A pilot signal is periodically transmitted ineach of the communications channel to facilitate the measurement of thechannel conditions. From the measured channel conditions, it is possibleto determine which channel has the best channel conditions from thewireless terminal's perspective at a particular point in time. Thewireless terminal provides channel condition information in messages tothe base station. This information is used for power and rate controland/or transmission scheduling purposes. In some embodiments, eachindividual wireless terminal feeds back channel condition informationand the base station selects, based on the channel conditioninformation, which channel to use to transmit information to thewireless terminal. The base station will normally select the channelwith the best conditions, e.g., highest SNR, from the plurality ofchannels for which a wireless terminal provides channel conditioninformation. If that best channel is not available, the base station mayselect the next best channel. To reduce the amount of informationrequired to be transmitted from a wireless terminal to the base stationon a recurring basis, in some embodiments the wireless terminals select,based on channel condition measurements of multiple channels, whichchannel is to be used for transmitting information to the wirelessterminal at a particular point in time. The wireless terminalcommunicates the channel selection as part of the channel feedbackinformation supplied to the base station on a periodic basis. In suchembodiments, the feedback information transmitted from a wirelessterminal to the base station normally includes a channel identifier andchannel quality information, e.g., a signal to noise ratio (SNR) or asignal to interference ratio (SIR).

The base station services many wireless terminals and, multiple wirelessterminals may select the same channel to be used to transmit informationduring the same time period. In cases where a communications channel hasbeen selected to be used by multiple wireless terminals, the basestation takes into consideration the channel quality reported by theindividual wireless terminals and gives a preference to the wirelessterminals reporting higher channel quality than those reporting lowerchannel quality. Other quality of service and/or fairness criterion isalso taken into account when the base station makes the schedulingdecision in at least some embodiments. Scheduling latency is reduced ascompared to systems using a single communications channel as a result ofusing multiple channels with differing physical characteristics whichare reflected in the channel quality reported by the wireless terminals.

In various embodiments channels are implemented as a partition of an airlink resource where each channel corresponds to a different portion ofthe air link resource in terms of time and/or frequency. To avoidrequiring a wireless terminal to switch between multiple carrierfrequencies, in some embodiments the carrier frequency used to transmitsignals to a wireless terminal is the same on the plurality of differentcommunications channels. In such an embodiment a wireless terminal canswitch between channels without having to change the frequency used tomix a received signal from the passband to the baseband as part of ademodulation process. This has the advantage of allowing for rapidswitching between communications channels which allows for switching tooccur without interfering with ongoing Internet Protocol sessions evenwhen the channel used to communication the voice or data packets ischanged during an ongoing IP communications session.

To provide for the ability to switch between channels on a rapid basis,in some embodiments, wireless terminals maintain channel qualityestimates and/or channel estimates for a plurality of differentcommunications channels at the same time. In such embodiments at leasttwo channel quality estimates and/or channel estimates are maintained atthe same time. The two channel estimates are normally for the two bestchannels to the wireless terminal, as determined by the wirelessterminal's measurements of the different channels. In some embodiment 3,4 or more channel estimates are maintained. Each of the channelestimates is usually maintained independent of the other channelestimates so that the individual channel estimate will properly reflectthe particular physical characteristics of the channel to which itcorresponds. Channel estimates are normally based on multiple channelmeasurements which occur at different points in time.

In some embodiments multiple static communications channels are used. Inat least one such embodiment at least 3 different channels are used.However the use of more channels with different physicalcharacteristics, e.g., 4, 8 or even more in a cell is possible.

While use of multiple static channels with differing characteristicsprovides scheduling advantages over embodiments where a single channelis used, even greater benefits can be obtained by introducing variationsinto one or more of the different communications channels.

In some embodiments, beamforming methods of the type described in U.S.patent application Ser. No. 09/691,766 filed Oct. 18, 2000 which ishereby expressly incorporated by reference, are used on individualchannels to deliberately induce channel variations. Multiple transmitterantennas are used in such an embodiment to facilitate introducingvariations into the communication channel. This method results inchannel variations that can be exploited by an opportunistic schedulersuch as that used in the base station of the present invention.

By combining the opportunistic beamforming method, e.g., theintroduction of intentional channel variations, with the use of multipleparallel communications channels, scheduling latency can be reducedbeyond the latency reduction benefits that can be achieved usingopportunistic beamforming alone. In fact, in some cases latency can bereduced by an amount directly related, if not proportional to, thenumber of different channels supported in the cell for communicationinformation to the wireless terminals. The reduction in latency can beto a level that would not be possible using a single channel andbeamforming since the rate at which beamforming can be used to change achannel in a productive manner is limited by the rate at which awireless terminal measures the channel and provides channel qualityinformation to a base station.

The use of parallel communications channels with multiple opportunisticbeams creates an improved version of transmit antenna diversity whichmay be exploited using channel selection by the wireless terminal and/orbase station based on channel quality measurements. Each of the parallelcommunications channels will normally exhibit a distinct wirelesschannel quality, thereby allowing the scheduler to take advantage of thediversity with a latency that will be a fraction of that possible when asingle channel is used.

In accordance with the present invention, in the case where intentionalvariations are introduced into a communications channel, the rate atwhich the channel variations occur is usually slower than the rate atwhich the wireless terminals measure the quality of the particularchannel which is being varied. In addition, the rate at which thewireless terminal provides channel feedback information, e.g., on asingle channel, is usually faster than the rate at which channels areintentionally varied. In such embodiments the periodicity of theintroduced channel variations is usually longer, e.g., in some cases atleast twice as long, as the rate at which quality measurements of theparticular channel are made and reported back to the base station. Insuch cases the relatively gradual change in the channel which isintentionally introduced should not have a significant impact on theaccuracy of the channel estimate maintained by the wireless terminal orthe channel condition information returned by a wireless terminal to abase station.

In order to reduce the possibility of repeated periods of interferenceaffecting the same wireless terminal, the rate at which channelvariations are introduced into channels of adjoining cells is controlledto be different. Thus, the base stations of adjoining cells, in someembodiments, introduce channel variations at different rates.

While the use of multiple transmission elements, e.g., multipleantennas, at a base station is not essential to the present invention,numerous embodiments of the present invention are implemented usingmultiple antennas. In some of these embodiments, control coefficientsets are maintained and used to control processing of signalstransmitted from a base station using different antennas. In suchembodiments, different antennas may be used for different communicationschannels. Alternatively, the same set of antennas can be shared by thedifferent communications channels with signal processing being used tointroduce amplitude and/or phase variations into the signalscorresponding to the different parallel communications channels. Theantenna pattern corresponding to a particular channel is varied in someembodiments to thereby vary the gain of the channel in a particulardirection. The gain of multiple channels may be changed in unison tomain a uniform difference between the channels to the extent possible.

The method and apparatus of the present invention may be used in a widerange of systems including frequency hopping, time division and/or codedivision based communications systems.

Numerous additional features and benefits are described in the detaileddescription which follows.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates an exemplary wireless communication systemimplemented in accordance with the invention.

FIG. 2 illustrates an exemplary cell of the communication system of FIG.1, exemplary communications channels, and exemplary signaling inaccordance with the present invention.

FIG. 3 illustrates an exemplary base station, suitable for use in thesystem of FIG. 1, implemented in accordance with the present invention.

FIG. 4 illustrates an exemplary wireless terminal, suitable for use inthe system of FIG. 1, implemented in accordance with the presentinvention.

FIG. 5 illustrates the construction of exemplary parallel pipes, using atime partition method, between a base station and wireless terminals, inaccordance with the invention.

FIG. 6 illustrates the construction of exemplary parallel pipes, using afrequency partition method, between a base station and wirelessterminals, in accordance with the invention.

FIG. 7 illustrates the construction of exemplary parallel pipes, using acombination of frequency division/time division methods, between a basestation and wireless terminals, in accordance with the presentinvention.

FIG. 8 illustrates exemplary parallel pipes using frequency division forexemplary 5 MHz CDMA/OFDM systems, in accordance with the presentinvention.

FIG. 9 illustrates exemplary parallel pipes in a 1.25 MHZ CDMA or OFDMsystem using time division, in accordance with the present invention.

FIG. 10 is a diagram of an exemplary transmitter using parallel pipesand multiple antennas, in accordance with the present invention.

FIG. 11 is a graph illustrating opportunistic beamforming for a singlebeam, in accordance with the present invention.

FIG. 12 is a graph illustrating opportunistic beamforming for twoexemplary beams in accordance with the present invention.

FIG. 13 illustrates the use of two exemplary downlink parallel pipes(constructed by frequency division) and uplink signaling includingchannel quality reports (including pipe selection by WTs), in accordancewith the present invention.

FIG. 14 illustrates a portion of an exemplary wireless communicationssystem showing an embodiment of the invention suited for applicationswhere channels are constructed using time division multiplexing.

FIG. 15 illustrates a portion of an exemplary wireless communicationssystem showing an embodiment of the invention suited for applicationswhere channels are constructed using frequency division multiplexing.

FIG. 16 is a drawing illustrating alternate pipes in alternate timeslots, in accordance with the invention.

FIG. 17 is a drawing illustrating parallel pipes during the same timeslots, in accordance with the invention.

FIG. 18 is a drawing illustrating four parallel pipes with differenttransmission characteristics which are varied over time.

FIGS. 19-22 show changes in antenna patterns over time, in accordancewith the present invention.

FIG. 23, which comprises the combination of FIGS. 23A, 23B, 23C, is aflowchart illustrating an exemplary method of operating a wirelesscommunications system in accordance with the present invention.

DETAILED DESCRIPTION

FIG. 1 is an illustration of an exemplary wireless communications system100, implemented in accordance with the present invention. Exemplarywireless communications system 100 includes a plurality of base stations(BSs): base station 1 102, base station M 114.

Cell 1 104 is the wireless coverage area for base station 1 102. BS 1102 communicates with a plurality of wireless terminals (WTs): WT(1)106, WT(N) 108 located within cell 1 104. WT(1) 106, WT(N) 108 arecoupled to BS 1 102 via wireless links 110, 112, respectively.Similarly, Cell M 116 is the wireless coverage area for base station M114. BS M 114 communicates with a plurality of wireless terminals (WTs):WT(1′) 118, WT(N′) 120 located within cell M 116. WT(1′) 118, WT(N′) 120are coupled to BS M 114 via wireless links 122, 124, respectively. WTs(106, 108, 118, 120) may be mobile and/or stationary wirelesscommunication devices. Mobile WTs, sometimes referred to as mobile nodes(MNs), may move throughout the system 100 and may communicate with thebase station corresponding to the cell in which they are located. Region134 is a boundary region between cell 1 104 and cell M 116.

Network node 126 is coupled to BS 1 102 and BS M 114 via network links128, 130, respectively. Network node 126 is also coupled to othernetwork nodes/Internet via network link 132. Network links 128, 130, 132may be, e.g., fiber optic links. Network node 126, e.g., a router node,provides connectivity for WTs, e.g., WT(1) 106 to other nodes, e.g.,other base stations, AAA server nodes, Home agents nodes, communicationpeers, e.g., WT(N′), 120, etc., located outside its currently locatedcell, e.g., cell 1 104.

FIG. 2 is a drawing 200 of cell 1 104 illustrating exemplarycommunications channels and exemplary signaling in accordance with thepresent invention. FIG. 2 includes communications within cell 1 104between BS 1 102 and WTs (WT(1) 106, WT(N) 108). BS 1 102 includesmultiple transmit antennas, e.g., transmitter antenna 1 202, transmitterantenna N 204. The base station 502 can transmit by multiple antennas202, 204 to each WT 106, 108.

In the illustration of FIG. 2, the two solid lines (206, 208), one fromeach antenna (202, 204) to WT(1) 106, represent a first pipe to WT(1)106. Similarly, the two dashed lines (210, 212), one from each antenna(202, 204) to WT(1) 106, represent a second pipe to WT(1) 106. Thus,solid lines (206,208) correspond to one set of communications signalswhich combine in the air to operate as one downlink communicationschannel to WT(1) 106, while dashed lines (210, 212) represent signalswhich combine in the air and operate as a second downlink communicationschannel to WT(1) 106.

Similarly, the two solid lines (214, 216), one from each antenna (202,204) to WT(N) 108, represent a first pipe to WT(N) 108; the two dashedlines (218, 220), one from each antenna (202, 204) to WT(N) 108,represent a second pipe to WT(N) 108. Thus, solid lines (214, 216)correspond to one set of communications signals which combine in the airto operate as one downlink communications channel to WT(N) 108, whiledashed lines (218, 220) represent signals which combine in the air andoperate as a second downlink communications channel to WT(N) 108. Fromthe perspective of each WT 106, 108 they are coupled to BS 1 102 by twoseparate pipes from which information may be received at any given time.The wireless terminals (106, 108) provide feedback information to basestation 1 102 as represented by arrows (222, 224) proceeding from eachWT (106, 108), respectively, to base station 102. Feedback signals tothe base station may include information on each of these pipes. Basedon this feedback information, the BS 102 may determine which pipe to useand when to transmit data to the WT(1) 106 and/or WT(N) 108. In someembodiments, each WT (106, 108) sends a signal to the BS 102 indicatingwhich of the pipes should be used at any point in time.

FIG. 3 illustrates an exemplary base station 300, implemented inaccordance with the present invention. Exemplary BS 300 may be a moredetailed representation of any of the BSs, BS 1 102, BS M 114 of FIG. 1.BS 300 includes a receiver 302, a transmitter 304, a processor, e.g.,CPU, 306, an I/O interface 308, I/O devices 310, and a memory 312coupled together via a bus 314 over which the various elements mayinterchange data and information. In addition, the base station 300includes a receiver antenna 216 which is coupled to the receiver 302.The base station 300, as shown in FIG. 3, also includes multipletransmitter antennas, (antenna 1 318, antenna n 322) which arephysically spaced apart from each other. Transmitter antennas 318, 322are used for transmitting information from BS 300 to WTs 400 (see FIG.4) while receiver antenna 216 is used for receiving information, e.g.,channel condition feedback information as well as data, from WTs 400.

The memory 312 includes routines 324 and data/information 326. Theprocessor 306, executes the routines 324 and uses the data/information326 stored in memory 312 to control the overall operation of the basestation 300 and implement the methods of the present invention. I/Odevices 310, e.g., displays, printers, keyboards, etc., display systeminformation to a base station administrator and receive control and/ormanagement input from the administrator. I/O interface 308 couples thebase station 300 to a computer network, other network nodes, other basestations 300, and/or the Internet. Thus, via I/O interface 308 basestations 300 may exchange customer information and other data as well assynchronize the transmission of signals to WTs 400 if desired. Inaddition I/O interface 308 provides a high speed connection to theInternet allowing WT 400 users to receive and/or transmit informationover the Internet via the base station 300. Receiver 302 processessignals received via receiver antenna 216 and extracts from the receivedsignals the information content included therein. The extractedinformation, e.g., data and channel condition feedback information, iscommunicated to the processor 306 and stored in memory 312 via bus 314.Transmitter 304 transmits information, e.g., data, and pilot signals toWTs 400 via multiple antennas, e.g., antennas 318, 322. Transmitter 304includes a plurality of phase/amplitude control modules, phase/amplitudecontrol module 1 316, phase/amplitude control module n 320. In theillustrated example of FIG. 3, a separate phase/amplitude controlmodule, (316, 320) is associated with each of the transmit antennas(318, 322), respectively. The antennas 318, 322 at the BS 300 are spacedfar enough apart so that the signals from the antennas 318, 322 gothrough statistically independent paths, and thus the channels thesignals go through are independent of each other. The distance betweenantennas 318, 322 is a function of the angle spread of the WTs 400, thefrequency of transmission, scattering environment, etc. In general, halfa wavelength separation between antennas, based on the transmissionfrequency, is usually the sufficient minimum separation distance betweenantennas, in accordance with the invention. Accordingly, in variousembodiments, antennas 318, 322 are separated by one half a wavelength ormore, where a wavelength is determined by the carrier frequency f_(k) ofthe signal being transmitted.

The phase and amplitude control modules 316, 320 perform signalmodulation and control the phase and/or amplitude of the signal to betransmitted under control of the processor 306. Phase/amplitude controlmodules 316, 320 introduce amplitude and/or phase variations into atleast one of a plurality, e.g., two, signals being transmitted to a WT400 to thereby create a variation, e.g., an amplitude variation overtime, in the composite signal received by the WT 400 to whichinformation is transmitted from multiple antennas 318, 322. The controlmodules 316, 320 are also capable of varying the data transmission rate,under control of the processor 306, as a function of channel conditionsin accordance with the present invention. In some embodiments,phase/amplitude control modules 316, 320 change phase and/or amplitudeby changing coefficients.

As mentioned above, the processor 306 controls the operation of the basestation 300 under direction of routines 324 stored in memory 312.Routines 324 include communications routines 328, and base stationcontrol routines 330. The base station control routines 330 include atransmit scheduler/arbitration module 332 and a receiverscheduler/arbitration module 334. Data/Information 326 includestransmission data 336 and a plurality of wireless terminal (WT)data/information 338. WT data/information 338 includes WT 1 information340 and WT N information 342. Each WT information set, e.g., WT 1information 340 includes data 344, terminal ID information 346, channelcondition information 348, and stored customer information 350. Storedcustomer information 350 includes modulation scheme information 352,transmission antenna information 354, and transmission frequencyinformation 356. Transmission data 336 includes data, e.g., user data,intended to be transmitted to WTs 400, located within the cell of BS300. Data 344 includes user data associated with WT 1, e.g., datareceived from WT 1 intended to be forwarded to a communication peer,e.g., WT N, and data receiver from a peer of WT 1, e.g., WT N, intendedto be forwarded to WT 1. Terminal ID information 346 includes a currentbase station assigned identity for WT 1. Channel condition information348 includes feedback information from WT 1 such as, e.g., downlinkchannel(s) estimation information and/or a WT 1 selected downlinkchannel.

The transmit scheduler/arbitration module 332 schedules whentransmission data 336 will be transmitted, e.g., downloaded, to WTs 400.As part of the scheduling process module 332 arbitrates between theneeds of various WTs 400 to receive data. The receiverscheduler/arbitration module 334 schedules when WTs 400 will be allowedto upload data to the BS 300. As with the transmit scheduler 332, thereceiver scheduler 334 may arbitrate between several WTs 400 seeking toupload data at the same time. In accordance with the present invention,modules 332, 334 perform scheduling operations as a function of receivedchannel condition feedback information, e.g., WT 1 channel conditioninformation 348. Communications routines 328 determine the frequency anddata rate as well as the appropriate encoding or modulation technique tobe used for communications with each WT 400. Communications routine 328can access the stored channel condition information and customerinformation, e.g., WT1 channel condition information 344 and WT 1 storedcustomer information 350 to obtain relevant information used by theroutines 324. For example, communications routines 328 can accesschannel condition information 348 obtained from feedback to determinethe appropriate data rate to be used in communicating to a WT 400. Inaddition, other stored customer information 350 such as modulationscheme information 352, transmission antenna information 354, andtransmission frequency information 356 can be retrieved and used todetermine the appropriate modulation scheme, number of transmissionantennas, and transmission frequency to be used when communicating witha particular WT 400 scheduled to receive information.

While in some embodiments a single antenna is used to transmitinformation to a WT 400, the use of multiple physically separatedantennas 318, 332 allows the same information to be transmitted fromdifferent locations with controlled phase and/or amplitude differencesbeing introduced into at least one of the transmitted signals to producean artificial signal variance at the receiving WT 400.

FIG. 4 illustrates an exemplary wireless terminal 400, implemented inaccordance with the present invention. Exemplary wireless terminal 400may be a more detailed representation of any of the WTs 106, 108, 118,120 of exemplary system wireless communication system 100 of FIG. 1. WT400 includes a receiver 402, a transmitter 404, I/O devices 406, aprocessor, e.g., a CPU, 408, and a memory 410 coupled together via bus412 over which the various elements may interchange data andinformation. Receiver 402 is coupled to antenna 414; transmitter 404 iscoupled to antenna 416. In some embodiments, a single antenna may beused in place of the two individual antennas 414 and 416.

Downlink signals transmitted from BS 300 are received through antenna414, and processed by receiver 402. Transmitter 404 transmits uplinksignals through antenna 416 to BS 300. Uplink signals include downlinkfeedback channel estimation information and/or information identifying aselected downlink channel over which WT 400 requests that downlink databe transmitted, in accordance with the invention. I/O devices 406include user interface devices such as, e.g., microphones, speakers,video cameras, video displays, keyboard, printers, data terminaldisplays, etc. I/O devices 406 may be used to interface with theoperator of WT 400, e.g., to allow the operator to enter user data,voice, and/or video directed to a peer node and allow the operator toview user data, voice, and/or video communicated from a peer node, e.g.,another WT 400.

Memory 410 includes routines 418 and data/information 420. Processor 408executes the routines 418 and uses the data/information 420 in memory410 to control the basic operation of the WT 400 and to implement themethods of the present invention. Routines 418 include communicationsroutine 422 and WT control routines 424. WT control routines 424 includea channel condition measurement module 426 and a channel selectionmodule 428.

Data/Information 420 includes transmission data 430, stored base stationinformation 432, and user information 434. User information 434 includesbase station identification information 436, terminal ID information438, assigned downlink channel information 440, a plurality of channelmeasurement information (channel 1 measurement information 442, channelN measurement information 446), a plurality of channel estimateinformation (channel 1 estimate information 444, channel N estimateinformation 448), and selected channel information 450. Transmissiondata 430 includes user data, e.g., data/information to be transmitted toBS 300 intended for a peer node in a communication session with WT 400,downlink channel feedback information, and/or a selected downlinkchannel. Stored base station information 432 includes informationspecific to each base station, e.g., slope values that may be used inhopping sequences, carrier frequencies used by different base stations,modulation methods used by different base stations, beamformingvariations that are base station dependent, etc. User information 432includes information being currently used by WT 400. Base station IDinformation 436 includes identification information of the base stationin whose cell WT 400 is currently located, e.g., a value of slope usedin a hopping sequence. Terminal ID information 438 is a base stationassigned ID used for current identification of WT 400 by the BS 300 inwhose cell WT is located. Assigned downlink channel information 440includes a downlink channel assigned by the BS 300 for the WT 400 toexpect user data to be transmitted on. Channel 1 measurement information442 includes measurements of received signals corresponding to channel1, e.g., measurements of a pilot signal transmitted on downlink channel1 such as SNR (Signal to Noise Ration), SIR (Signal Interference Ratio),etc. Channel N measurement information includes measurement of receivedsignals corresponding to channel N, e.g., measurements of a pilot signaltransmitted on downlink channel N such as SNR, SIR, etc. Channel 1estimation information 444 includes downlink channel 1 estimates, e.g.,based on channel 1 measurement information 442. Channel N estimationinformation 448 includes downlink channel 2 estimates based on channel Nmeasurement information 446. Selected channel information 450 includesinformation identifying which channel WT 400 has identified as the moredesirable downlink channel, e.g., which of the beam formed downlinkchannels 1, N is better suited at the present time for WT 400. Selectedchannel information 450 may also include channel measurement informationcorresponding to the selected channel.

The communications routine 422 controls the transmission and receptionof data by transmitter 404 and receiver 402, respectively.Communications routine 422 may vary the data transmission rate, inaccordance with the present invention based on channel conditions. Inaddition, communications routine 422 is responsive to schedulinginformation, received from BS 300 to insure that transmission data 430is transmitted by the WT 400 at the times authorized by the BS 300.Communications routines 422 transmits channel condition information,e.g., channel measurement information 442, 446, selected channelinformation 450, and/or amplitude/phase feedback information to the BS300 via transmitter 404. Communications routines 422 are alsoresponsible for controlling the display and/or audio presentation ofreceived information to a WT user via I/O devices 406.

Channel condition measurement module 426 measures channel conditionsobtaining channel 1 measurement information 442, channel N measurementinformation 446. Channel condition measurement module 426 also processesthe channel measurement information 442, 446 and obtains channelestimate information 444, 448, respectively. Channel conditionmeasurement module 426 also supplies the amplitude and/or phase feedbackinformation to the communications routine 422. Channel selection module428 compares channel measurement information, e.g., channel 1measurement information 442, channel N measurement information 446,selects which channel is better, stores the selection in selectedchannel information 450, and supplies the selected channel information450 to the communications routine 422. Communications routine 422 thentransmits channel measurement information 442, 446, selected channelinformation 450, and/or amplitude/phase information to the BS 300 viatransmitter 404.

FIG. 5 illustrates an exemplary embodiment of the construction ofparallel pipes, e.g., downlink channels between BS 300 and WT 400. Inthe time partition method of FIG. 5, the time is divided into parallelpipes, each of which can be used simultaneously to transmit signalsduring a different time slot but using the same bandwidth. FIG. 5 is agraph 500 of frequency on the vertical axis 502 vs time on thehorizontal axis 504. The air link resource represented by box 506 ispartitioned in time into an exemplary four parallel pipes 508, 510, 512,514. In the time partition method, each of the parallel pipes 508, 510,512, 514 occupies the entire bandwidth 516 but within different timeslots 518, 520, 522, 524.

FIG. 6 illustrates another exemplary embodiment of the construction ofparallel pipes, e.g., downlink channels between BS 300 and WT 400. Inthe frequency partition method of FIG. 6, the bandwidth is divided intoparallel pipes, each of which can be used simultaneously to transmitsignals in parallel. FIG. 6 is a graph 600 of frequency on the verticalaxis 602 vs time on the horizontal axis 604. The air link resourcerepresented by box 606 is partitioned in frequency into an exemplaryfive parallel pipes 608, 610, 612, 614, 616. In the frequency partitionmethod, each of the parallel pipes 608, 610, 612, 614, 616 occupies adifferent frequency range 618, 620, 622, 624, 626 but occupies theentire time slot 628.

FIG. 7 illustrates another embodiment of the construction of parallelpipes, e.g., downlink channels between BS 300 and WT 400. The FIG. 7embodiment combines the above embodiments of frequency division method(FIG. 6) and time division method (FIG. 5) to construct parallel pipes.FIG. 7 is a graph 700 of frequency on the vertical axis 702 vs time onthe horizontal axis 704. The air link resource represented by box 706 issubdivided into 12 parallel pipes 708, 710, 712, 714, 716, 718, 720,722, 724, 726, 728, 730.

FIG. 8 and FIG. 9 illustrate exemplary embodiments of using parallelpipes in exemplary CDMA and OFDM systems. FIG. 8 illustrates parallelpipes in exemplary systems using frequency division. In FIG. 8, drawing850 shows frequency on the horizontal axis 802 corresponding to anexemplary CDMA system having a 5 MHz bandwidth 804 in total, which ispartitioned into three carriers 806, 808, 810 each representing a 1.25MHz pipe 810, 812, 814. Thus, there are three parallel pipes, pipe 1810, pipe 2 812, and pipe 3 814 in that 5 MHz CDMA system. Drawing 850shows frequency on the horizontal axis 852 corresponding to an exemplaryOFDM system also having a 5 MHz bandwidth 854 in total, which is dividedinto N tones 853. In the figure, those N tones are grouped into foursubsets, pipe 1 856, pipe 2 858, pipe 3 860, pipe 4 862. Thus, there arefour parallel pipes 856, 858, 860, 862 in that 5 MHz OFDM system.

FIG. 9 is a graph 900 of frequency on the vertical axis 902 vs time onthe horizontal axis 904. The exemplary illustrated CDMA or OFDM systemrepresented by FIG. 9 has a 1.25 MHz bandwidth 906 in total, which isshared by two parallel pipes 908, 910 in a time division manner. Infirst time slot 912 (t=t₀ to t=t₁), pipe 1 908 is used; in second timeslot 914 (t=t₁ to t=t₂) pipe 2 910 is used; in third time slot 916 (t=t₂to t=t₃) pipe 1 908 is used; in fourth time slot 918 (t=t₃ to t=t₄) pipe2 910 is used.

In various embodiments of the present invention, the bandwidth, numberof pipes, number of carriers, number of tones, and/or number of subsetsmay vary. In various embodiments of the present invention, the partitionallocation for each pipe may vary.

In accordance with the invention WT 400, under the control of channelcondition measurement module 426, controls receiver 402 to measurereceived signals in order to obtain the channel quality of each of theparallel pipes. Channel (1,N) measurement information (442, 446) isobtained from the received signal. Separate channel measurements ofmultiple parallel pipes allows the WT 400 to perform pipe selection. Thechannel (1,N) measurement information (442, 446) may includesignal-to-interference ratio (SIR) and fading characteristics. Eachparallel pipe may have its own pilot(s) to facilitate the channelquality measurement, and the densities of pilots used may depend on thepartitioning of the air link resource.

The WT 400 then reports the measurement results back to the transmissionsource, BS 300. In some embodiments, the reporting is frequent and/orperiodic. In one embodiment, the channel quality report includes a listof the measurements of channel qualities in individual parallel pipes,e.g., channel (1,N) measurement information (442, 446). In anotherembodiment, the channel quality report includes the index of one of theparallel pipes that has the best channel quality and the correspondingchannel quality measurement, e.g., selected channel information 450.

In accordance with the invention, for a wireless system, e.g., system100 equipped with multiple transmitter antennas 318, 322 at the basestation 300, the antennas 318, 322 are used to create differentopportunistic beams for different parallel pipes. For the sake ofdescription, consider the case of two antennas. The same principle canbe easily extended to the case of many antennas. Let K denote the numberof parallel pipes.

Denote the signal to be transmitted at time instant t over the Kparallel pipes asS (t)={S ₁(t),S ₂(t), . . . , S _(K)(t)}(Note: In some locations vectors are notated by, lines above the symbol,in other locations vectors are denoted by underlining and/or boldfaceprint. These conventions may be used interchangeably throughout thisapplication.)

In an exemplary general description of the invention, two signals arederived from this basic signal and transmitted over the two transmitantennas respectively. The two derived signals may be described asS ⁽¹⁾ (t)={c ₁(t)S ₁(t),c ₂(t)S ₂(t), . . . , c _(K)(t)S _(K)(t)}S ⁽²⁾ (t)={d ₁(t)S ₁(t),d₂(t)S ₂(t), . . . , d_(K) (t)S _(K)(t)}where c_(k) (t) and d_(k) (t) are, in general, complex time-varyingcoefficients superposed on the signal on the k-th parallel pipes overthe first and second transmit antenna, respectively. In accordance withthe invention, coefficients {c₁(t), c₂ (t), . . . , c_(K)(t)} and{d₁(t), d₂ (t), . . . , d_(K) (t)} are independent of the transmittedsignal S(t).

FIG. 10 illustrates a diagram 1000 of an exemplary embodiment of theinvention using multiple transmit antennas (1002, 1004) transmittingover parallel pipes. FIG. 10 shows k parallel pipes and two antennas.Pipe 1 component 1006, pipe 2 component 1008, . . . , and pipe k 1010correspond to antenna 1 1002. Pipe 1 component 1012, pipe 2 component1014, . . . , and pipe k 1016 correspond to antenna 2 1004.

Input signal S₁(t) 1018 is multiplied, via multiplier 1020 by complextime-varying coefficient c₁(t) 1022 generating pipe 1 component 1006;pipe 1 component 1006 is input to combining device 1024. Input signalS₂(t) 1026 is multiplied, via multiplier 1028 by complex time-varyingcoefficient c₂(t) 1030 generating pipe 2 component 1008; pipe 2component 1008 is input to combining device 1024. Input signal S_(k)(t)1032 is multiplied, via multiplier 1034 by complex time-varyingcoefficient c_(k)(t) 1034 generating pipe k component 1010; pipe kcomponent 1010 is input to combining device 1024. Input signal S₁(t)1018 is multiplied, via multiplier 1038 by complex time-varyingcoefficient d₁(t) 1040 generating pipe 1 component 1012; pipe 1component 1012 is input to combining device 1042. Input signal S₂(t)1026 is multiplied, via multiplier 1044 by complex time-varyingcoefficient d₂(t) 1046 generating pipe 2 component 1014; pipe 2component 1014 is input to combining device 1042. Input signal S_(k)(t)1032 is multiplied, via multiplier 1048 by complex time-varyingcoefficient d_(k)(t) 1050 generating pipe k component 1016; pipe kcomponent 1016 is input to combining device 1042.

The circuitry illustrated in FIG. 10 may be, e.g., part of transmitter304 in base station 300. In the FIG. 10 example a combining device(1024, 1042) is used to combine signals from various pipes fortransmission using an antenna. Each of the illustrated combing devicestakes signals being transmitted over parallel ‘pipes’ and processes themto generate a signal to be transmitted over a single physical antenna.Combining device 1024 takes pipe 1 component 1006, pipe 2 component1008, . . . pipe k component 1010 and combines them into signal S¹(t)1052 which is transmitted over antenna 1 1002. Combining device 1042takes pipe 1 component 1012, pipe 2 component 1014, . . . pipe kcomponent 1016 and combines them into signal S²(t) 1054 which istransmitted over antenna 2 1004. In the event of pipes created in thetime domain, the combining devices 1024, 1042 may be implemented asmultiplexers. For frequency-domain pipes, the combining devices 1024,1042 may be implemented as ‘summers’ since it is combines signals thatbelong to different frequency bands.

The invention results in transmit diversity gains being realized in thereceiver 402 of WT 400. Denote the channel responses from the twoantennas to the receiver as h_(c)(t) and h_(d) (t) respectively. For thesake of description, it is assumed that the channel response from anyantenna 318, 322 (in BS 300) to the receiver 402 (in WT 400) is constantacross frequency. However, this assumption does not diminish orconstrain the invention in any way. Therefore, the signal received bythe receiver 402 (in WT 400) is given byR (t)={[c ₁(t)h _(c)(t)+d ₁(t)h_(d)(t)]S ₁(t), . . . , [c_(K)(t)h_(c)(t)+d _(K)(t)h _(d)(t)]S _(K)(t)},where the k-th element in vector R(t) is the received signal over thek-th parallel pipe. Hence, when the invention is applied to the systemwith two transmit antennas and multiple parallel pipes, the compositechannel response in k-th parallel pipe from the transmitter to thereceiver is effectively given by c_(k)(t)h_(c)(t)+d_(k)(t)h_(d)(t). Witha suitable choice of the values of the coefficients {c_(k)(t)} and{d_(k)(t)} at the transmitter 304 (in BS 300), at least one pipe shouldhave decent composite channel quality with high probability, althoughthe composite channel responses of other pipes may be of bad quality. Inany event, the latency experienced by a receiver 402 (in WT 400) inwaiting for a time instant when it experiences high channel quality isdrastically reduced since it can select between opportune schedulinginstants on multiple pipes.

The idea of the opportunistic beamforming paradigm is that thetransmitter 304 (in BS 300) chooses proper values of the coefficients,the receiver 402 (in WT 400) independently measures the channelqualities of the parallel pipes. WT 400 reports to the BS 300 (withtransmitter 304) the measurement results, and the BS 300 controls thetransmitter 304 to send traffic to the receiver 402 with those pipesthat have good channel quality. To use the invention, the receiver 402does not need to estimate h_(c)(t) and h_(d)(t) explicitly.

In one of the embodiments of this invention, each of the parallel pipeshas its own opportunistic beam. FIG. 11 is a graph 1100 illustratingopportunistic beamforming for a single beam. FIG. 11 plots received SNRon the vertical axis 1102 vs time in slots on the horizontal axis 1104;the characteristic of the single opportunistic beam 1106 correspondingto a single parallel pipe is shown. FIG. 12, is a graph 1200illustrating opportunistic beamforming for two exemplary beams. FIG. 12plots received SNR on the vertical axis 1202 vs time in slots on thehorizontal axis 1204; the characteristic of the opportunistic beam 11206 corresponds to a first parallel pipe, while the characteristic ofopportunistic beam 2 1208 corresponds to a second parallel pipe. Thecomplex time-varying weights are adjusted so that the beams areeffectively offset from one another. The receiver 402 sees the channelquality varying over time on any particular pipe. In general, thereceiver 402 perceives high channel quality on one of the pipes (andcorresponding beams) when another pipe (and corresponding beam) offerlow channel quality, as illustrated in FIG. 12. It is easy to see thatusing two beams effectively reduces the latency at the receiver 402 inwaiting for a time instant when the channel quality is high and thereceiver 402 can select between the beams depending on their channelqualities. The receiver 402 is in a position to select the strongestamong these rotating beams and report the pipe associated with theselected beam (and the corresponding channel quality) to the transmitter304), such that the transmitter 304 can send traffic to the receiver 402with the pipe of the best channel quality.

In the present invention, with multiple rotating beams being transmittedon parallel pipes, the receiver 402 can see diverse channel quality in ashort time period and therefore the latency in getting good channelquality is significantly reduced.

The choice of the coefficients {c_(k)(t), d_(k)(t)} is quite flexible.In one embodiment, {c_(k)(t)} is set to a constant, {d_(k)(t)} is set tobe a constant-amplitude complex number with phase being rotating withtime, and the phase components of {d_(k)(t)} are uniformly with time:c _(k)(t)=1d _(k)(t)=exp(j2πft+v _(k))where the phase offsets {v_(k)} are uniformly distributed in [0,2π]. Forexample, for K=3,

${{\upsilon_{1} = 0},{\upsilon_{2} = \frac{2\pi}{3}},{\upsilon_{3} = \frac{4\pi}{3}},\mspace{14mu}{{and}\mspace{14mu}{for}}}\mspace{14mu}$${K = 4},{\upsilon_{1} = 0},{\upsilon_{2} = \frac{\pi}{2}},{\upsilon_{3} = \pi},{\upsilon_{4} = {\frac{3\pi}{2}.}}$This particular embodiment results in multiple opportunistic beams thateach rotates with frequency f.

As a special case of the embodiment, f can be zero, that is, theopportunistic beams do not rotate. In this case, the coefficients can bechosen in either a random manner, or with the phases uniformlydistributed, and can be held constant over at least some time period.This special case is especially attractive when a large number ofparallel pipes (K>2) are realized. Given the large number of parallelpipes, it is highly likely that at any given time, the receiver 402 canfind at least one pipe that is ‘highly beamformed’.

As a generalization to the embodiment, the coefficients can usedifferent and time-varying amplitudes:c _(k)(t)=√{square root over (α_(k)(t))}d _(k)(t)=√{square root over (1−α_(k)(t))}exp(j2πft+v _(k))where {α_(k) (t)} are real numbers.

In general, the number of pipes formed need not be the same as thenumber of opportunistic beams realized using multiple antennas. Multiplebeams (up to the number of transmit antennas) can be realized within thesame pipe, with the receivers tracking the signal quality on each ofthese beams on each of these pipes. In fact, different users can then bescheduled on the different beams within a pipe. For example, in the caseof two beams within a pipe, one user may have a null on the first beamand be scheduled on the second beam. Another user may be in acomplementary situation, having a null on the second beam and willtherefore be scheduled on the first beam.

When the pipes are formed by splitting the bandwidth and the totalsystem bandwidth is larger than a coherence bandwidth, the method ofbeam selection described here can exploit the diversity gains from boththe transmit antenna diversity and frequency diversity available in thesystem without requiring any scheduling latency.

In a cellular environment, the channel quality is determined not only bythe signal component but also by the interference component. To optimizethe channel quality, multiple transmit antennas and parallel pipes canbe used such that the receiver 402 is highly beamformed in its desiredcell, e.g., cell 1 104 (opportunistic beamforming) and at the same timehighly nulled in its adjacent cells, e.g., cell M 116 (opportunisticnulling). In one embodiment of the invention, each cell canindependently apply the invention illustrated in the above descriptionexcept that the frequency of rotation of beams f used in adjacent cellsmay be different.

FIG. 13 illustrates the use of two parallel pipes, indexed as 1 and 2,constructed by frequency division in a frequency division multiplexedsystem, e.g., an OFDM system. Graph 1300 illustrates downlink frequencyon the vertical axis 1302 vs time on the horizontal axis 1304. Thedownlink frequency is subdivided into pipe 1 1306 and pipe 2 1308. Eachbox 1310 in graph 1300 represents a downlink traffic pipe segment. FIG.1350 illustrates uplink signaling, e.g., downlink channel qualityreports, from three exemplary WTs 400 (WT A, WT B, WT C) to BS 300, inaccordance with the invention.

WTs 400 (A, B, C) including their respective receivers 402 (A, B, C),measure and estimate the channel quality of each of the parallel pipesusing the pilots transmitted by BS 300 in downlink signaling in thosepipes. The WTs 400 (A, B, C) then report back the best channel qualityvalue and the associated parallel pipe index, in their respectivechannel quality reports 1352, 1354, 1356. In this example, theopportunistic beamforming is such that the channel quality (SIR)measured by receiver A for the two pipes are 0 dB and 10 dB, the SIRmeasured by receiver B for the two pipes are 5 dB and −3 dB, and the SIRmeasured by receiver C for the two pipes are 0 dB and −2 dB. Therefore,WT A reports that the pipe of index 2 has the best channel quality andthe SIR is 10 dB, WT B reports that the pipe of index 1 has the bestchannel quality and the SIR is 5 dB, and WT C reports that the pipe ofindex 1 has the best channel quality and the SIR is 0 dB. Then, the BS300, including transmitter 304, decides to transmit a segment of traffic1312 to WT A using the pipe 2, and in parallel, to transmit anothersegment of traffic 1314 to receiver B using the pipe 1. The BS 300further determines the coding/modulation rate and transmission power tobe used in those two segments on the basis of the SIR reports from WTs Aand B. A short time later, WTs 400 (A, B and C) send their channelquality reports 1358, 1360, 1362, respectively, again. This time, WT Areports that the pipe of index 1 has the best channel quality and theSIR is 3 dB, WT B reports that the pipe of index 1 has the best channelquality and the SIR is 10 dB, and WT C reports that the pipe of index 2has the best channel quality and the SIR is 6 dB. Then, the base station300 decides to transmit a segment of traffic 1316 to WT B using the pipe1, and in parallel, to transmit another segment of traffic 1318 to WT Cusing the pipe 2.

Pipes discussed in the present invention represent channels which can beused to communicate information. Different pipes, e.g., differentchannels, will have intentionally induced channel variations. These perchannel variations can be measured by a wireless terminal 400. Theinduced channel variations will be reflected in channel feedbackreports. In various embodiments, the rate at which measurable channelvariations are introduced is the same as or slower than the channelreport feedback rate. In this manner, the BS 300 should have accuratechannel information which may not be the case if the period of channelvariations is shorter than the feedback report period.

Various features and embodiments of the present invention will now bediscussed further. FIGS. 14 and 15 show exemplary base stations whichcan be used to implement the methods discussed below. FIG. 14 shows aportion of an exemplary communications system 1400 including anexemplary base station (BS) 1402 and two exemplary wireless terminals,WT1 1404 and WT2 1406. BS 1402 includes an exemplary input signal S_(m)1409, coefficients 1407, a coefficient control module 1408, atransmitter module 1412, an a plurality of antennas (A₁ 1416, A₂ 1418, .. . , A_(k) 1420). The coefficient control module 1408 includescoefficient sets 1410 for a plurality of pipes (e.g., for pipes 1 to n).The transmitter module 1412 includes k processing elements (1422, 1424,. . . , 1426) corresponding to the k antennas (1416, 1418, . . . ,1420), respectively. The coefficient set for exemplary pipe m is shownwhere g _(m)=[g_(m,1), g_(m,2), . . . g_(m,k)]^(T). In base station1402, different sets of transmission coefficients 1410 are used togenerate different pipes, e.g., at alternating times. (See FIG. 16.) Forexample at the time when it is desired to transmit over pipe 1, S_(m)=S₁and g _(m)=g ₁=[g_(1,1), g_(1,2), . . . , g_(1,k)]^(T); at the time whenit is desired to transmit over pipe 2, S_(m)=S₂ and g _(m)=g₂=[g_(2,1),g_(2,2), . . . , g_(2,k)]^(T). One exemplary pipe 1403 isshown from BS 1402 to WT1 1404; a second exemplary pipe 1405 is shownfrom BS 1402 to WT2 1406. The coefficients control processing elements(1422, 1424, 1426), may be, e.g., gain and/or phase adjusting circuits.The FIG. 14 embodiment is well suited for cases where different channelsare constructed using time divisional multiplexing, e.g., CDMAapplications.

FIG. 15 shows a portion of an exemplary communications system 1500including an exemplary base station (BS) 1502 and two exemplary wirelessterminals, WT1 1504 and WT2 1506. BS 1502 includes an input signal S_(m) 1508, coefficients 1510, a coefficient control module 1512 atransmitter module 1514 an a plurality of antennas, (e.g., k antennas,A₁ 1516, A₂ 1518, . . . , A_(k) 1520). The coefficient control module1512 includes coefficient sets 1522 for a plurality of pipes (e.g., forpipes 1 to n). FIG. 15 illustrates an exemplary two pipe embodiment;other numbers of pipes are possible in accordance with the invention.The transmitter module 1514 includes a pipe control module for eachpipe, e.g., pipe 1 control module 1524, pipe 2 control module 1526.Transmitter module 1514 also includes k summing elements (1528, 1530, .. . , 1532) corresponding to the k antennas (1516, 1518, . . . , 1520),respectively. Each pipe control module (1524, 1526) includes kprocessing elements ((1534, 1536, . . . , 1538 for pipe 1), (1534′,1536′, . . . , 1538′ for pipe 2)) corresponding to the k antennas (1516,1518, . . . , 1520), respectively. The coefficient set for pipe 1 is g₁=[g_(1,1), g_(1,2), g_(1,k)]². The coefficient set for pipe 2 is g₂=[g_(2,1), g_(2,2), g_(2,k)]^(T) Input signal S _(m) 1508 includes a S₁component 1540 and an S₂ component 1521. S₁ input signal component 1540is the input signal to pipe 1 control module 1524; S₂ input signalcomponent 1542 is the input signal to pipe 2 control module 1526.

BS 1502, as shown in FIG. 15, is suitable for transmitting usingmultiple pipes in parallel where the different pipes may correspond todifferent sets of tones, e.g., frequencies. The FIG. 15 example isparticularly well suited for the case where the channels are constructedusing frequency division multiplexing, e.g., OFDM applications.

FIG. 16 is a drawing 1600 illustrating alternate pipes A and B (1602,1604) generated by using alternating sets of transmission controlcoefficients, e.g., using the transmitter shown in FIG. 14 and changesin coefficient sets over time 1606. The difference between channelcharacteristics, e.g., gain, normally differs between channels A and Bin any two adjacent slots more than the change in gain introduced in achannel between consecutive time slots used by a particular channel. Forexample, a large difference is maintained between channels A and B atany given time, while the individual channel A varies slowly over timeand individual channel B varies slowly over time.

FIG. 17 is a drawing 1700 illustrating parallel pipes A and B (1702,1704) over time 1706. Parallel pipes A and B (1702, 1704) are generatedusing first and second sets of coefficients, e.g., using the transmittershown in FIG. 15. Changes in coefficient sets are made over time toinduce channel variations. Differences between channel characteristics,e.g., gain, normally differ between channels A and B in any two parallelchannels more than the change in gain introduced in a channel betweenconsecutive time slots used by the particular channel. For example, alarge difference is maintained between channels A and B at any giventime, while individual channel A is varied slowly over time andindividual channel B is varied slowly over time.

FIG. 18 is a drawing 1800 illustrating four parallel pipes (pipe A 1802,pipe B 1804, pipe C 1806, pipe D 1808) with different transmissioncharacteristics which are varied over time, e.g., which are changed bymodifying transmission control coefficients at the end of eachtransmission time period (t_(i)). Four transmission periods t₁ 1812, t₂1814, t₃ 1816, and t₄ 1818 and their corresponding end points 1813,1815, 1817, and 1819, respectively, are shown.

FIGS. 19, 20, 21 and 22 show changes in antenna patterns over time inaccordance with the present invention as induced by using differenttransmission control coefficients over time for the different pipes,e.g., parallel or alternating channels. While shown as a single fixedantenna pattern during each illustrated time period it is to beunderstood that the pattern could be changed gradually during the timeperiod resulting in the pattern changing from that shown in one figureto that shown in the next figure by the conclusion of the particulartime period.

FIG. 19 illustrates an exemplary base station 1902 and an exemplary WT1904, implemented in accordance with the present invention. In FIG. 19 acombined antenna pattern is shown including antenna patterns 1906, 1908,1910, 1912 corresponding to channels A, B, C, D, respectively. Note eachlobe 1906, 1908, 1910, 1912 corresponds to the directional pattern ofone channel during illustrated time period T1 1901.

FIG. 20 illustrates the exemplary base station 1902 and the exemplary WT1904. In FIG. 20 a combined antenna pattern is shown including antennapatterns 2006, 2008, 2010, 2012 corresponding to channels A, B, C, D,respectively. Note each lobe 2006, 2008, 2010, 2012 corresponds to thedirectional pattern of one channel during illustrated time period T22001.

FIG. 21 illustrates the exemplary base station 1902 and the exemplary WT1904. In FIG. 21 a combined antenna pattern is shown including antennapatterns 2106, 2108, 2110, 2112 corresponding to channels A, B, C, D,respectively. Note each lobe 2106, 2108, 2110, 2112 corresponds to thedirectional pattern of one channel during illustrated time period T32101.

FIG. 22 illustrates the exemplary base station 1902 and the exemplary WT1904. In FIG. 22 a combined antenna pattern is shown including antennapatterns 2206, 2208, 2210, 2212 corresponding to channels A, B, C, D,respectively. Note each lobe 2206, 2208, 2210, 2212 corresponds to thedirectional pattern of one channel during illustrated time period T42201.

Note that the difference between the patterns is designed to minimizethe time before a wireless terminal 1904, e.g., mobile, located anywherein the 360 degree transmission field will have to wait beforeencountering a channel with an optimal or near optimal transmissionpattern which, as can be appreciated, will produce good channeltransmission characteristics from the wireless terminal's, e.g., mobilenodes, perspective. As discussed previously, the BS 1902, in accordancewith the invention, includes a transmit scheduler/arbitration module,(See, e.g., module 332 of FIG. 3) and uses channel feedback informationto schedule transmissions to individual wireless terminals.

FIG. 23, which comprises the combination of FIGS. 23A, 23B, and 23C, isa flowchart illustrating an exemplary method 2300 of operating awireless communications system in accordance with the present invention.The method begins with start node 2302, and operation proceeds to step2304. In step 2304 first and second base stations and wirelessterminals, e.g., mobile nodes, are initialized. For the exemplarywireless node, operation proceeds from step 2304 to step 2310. For theexemplary first base station, operation proceeds from step 2304 viaconnecting node B 2306 to step 2326. For the exemplary second basestation, operation proceeds from step 2304 via connecting node C 2308 tostep 2340.

In step 2310, the first wireless terminal in a first cell is operated tomeasure the quality of each of a plurality of different communicationschannels. Operation proceeds from step 2310 to step 2312. In step 2312,the first wireless terminal is operated to periodically report onmeasured channel quality on one or more of the different communicationschannels to the first base station. Operation proceeds to step 2314. Instep 2314, the first wireless terminal is operated to maintain aplurality of channel estimates and/or channel quality estimates inparallel for use in processing information signals received from saidfirst base station. Channel estimates are normally based on multiplemeasurements of the channel to which the particular estimatecorresponds, In step 2316, the first wireless terminal is operated toselect, based on channel quality measurements, the best one of thedifferent communications channels as perceived by the first wirelessterminal Operation proceeds from step 2316 to step 2318. In step 2318,the first wireless terminal is operated to periodically transmit afeedback signal to the first base station indicating the selectedchannel to be used to transmit information to the first wirelessterminal and information on the quality of the selected channel, e.g.,the SNR and/or SIR of the selected channel, the rate of feedbacksignaling being the same as or faster, e.g., 2×, the rate at which thefirst base station changes signal transmission characteristics. In step2320, the first wireless terminal is operated to receive information onthe selected channel after the first base station switches from a firstchannel to a selected channel when transmitting information to the firstwireless terminal in response to the feedback information. Operationproceeds from step 2320 to step 2322. In step 2322, the first wirelessterminal is operated to switch between a first channel estimate and achannel estimate corresponding to the selected channel in response toreceiving information on the selected channel. In step 2324, the firstwireless terminal is operated to demodulate the information received onthe selected channel by performing a passband to baseband conversionoperation.

In step 2326, the first base station in the first cell is operated totransmit signals on a plurality of different communications channels,each individual one of the plurality of different communicationschannels each having a physical characteristic which is detectable bythe first wireless terminal, a pilot signal being transmitted on aperiodic basis on each channel, information to individual wirelessterminals, e.g., corresponding to a communications session, beingtransmitted according to a schedule. Step 2326 includes sub-step 2328.In sub-step 2328, the first base station is operated to periodicallychange at least one signal transmission characteristic of each of saidplurality of communications channels by modifying one or morecoefficients used to control the signals transmitted using multipleantennas, said changing occurring at a rate equal to or slower than arate at which channel condition feedback information is received from awireless terminal Operation proceeds to step 2330. In step 2330, thefirst base station is operated to receive feedback information from aplurality of wireless terminals to which said first base stationtransmits signals, said feedback information including feedbackinformation from the first wireless terminal, said first wirelessterminal feedback information including information indicating thequality at said first wireless terminal of one or more channels and insome embodiments a channel selected by said first wireless terminal fortransmission of information to said first wireless terminal; saidfeedback information further including information from a secondwireless terminal, said second wireless terminal feedback informationincluding information indicating the quality at said second wirelessterminal of one or more channels, and in some embodiments, a channelselected by said second wireless terminal for transmission ofinformation to said second wireless terminal Operation proceeds fromstep 2330 to step 2332. In step 2332, the first base station is operatedto select between the plurality of communications channels to use totransmit information to the first and second wireless terminals, saidfirst base station selecting the channel for purposes of transmitting tothe first wireless terminal a channel identified in received feedbackinformation as having been selected by the first wireless terminal orthe channel indicated by the feedback information from the firstwireless terminal as having the best transmission characteristics, saidselecting resulting in a switching between channels if a selectedchannel differs from a channel which is currently being used to transmitinformation to a wireless terminal Operation proceeds from step 2332 tostep 2334. In step 2334, the first base station is operated to scheduleinformation transmissions to individual wireless terminals as a functionof the channel selected for transmitting to the individual wirelessterminals, said scheduling including giving priority to wirelessterminals to use a channel which reported better channel conditions thanother wireless terminals selected to use the same channel. Operationproceeds to step 2336; in step 2336 the first base station is operatedto transmit information to the wireless terminals at the scheduled timesusing the selected channels. From step 2336 operation proceeds viaconnecting node D 2338 to step 2330.

In step 2340, the second base station is operated in a second cellphysically adjoining said first cell to transmit signals on a pluralityof different communications channels in the second cell each individualone of the plurality of different communications channels in the secondcell having a physical characteristic which is detectable by a firstwireless terminal in the second cell, a pilot signal being transmittedon a periodic basis on each channel, information to individual wirelessterminals, e.g., corresponding to a communications session, beingtransmitted according to a schedule. Step 2340 includes sub-step 2342.In sub-step 2342, the second base station is operated to periodicallychange at least one signal transmission characteristic of each of saidplurality of communications channels in the second cell by modifying oneor more coefficients used to control the signals transmitted usingmultiple antennas, said changing occurring at a rate equal to or slowerthan a rate at which channel condition feedback information is receivedfrom a wireless terminal, said changing occurring at a rate which isdifferent from the rate at which the said first base stationperiodically changes at least one signal transmission characteristic.Operation proceeds to step 2344. In step 2344, the second base stationis operated to receive channel condition feedback information fromwireless terminals in the second cell, select channels to transmitinformation to said wireless terminals and to schedule informationtransmissions. Operation proceeds from step 2344 to step 2346. In step2346, the second base station is operated to transmit information towireless terminals in the second cell at scheduled times using selectedchannels. Operation proceeds from step 2346 to step 2344.

A method of the design of beamforming coefficients, in accordance withthe invention will now be discussed. A particular design method oftime-varying beamforming coefficients, g _(m)(t) will be discussed.(Note: underlining is used to connote a vector.) First the design willbe considered for a single pipe case, then it will be extended tomultiple pipes.

Intuitively, the beamforming coefficient vector should, over time,“sweep” over a large range of possible channel gains such that g(t) willperiodically come close to the optimal beamforming configuration foreach user. In general, it is advantageous to vary both the phase andmagnitude of the coefficients of the K antenna gains thereby producing amultidimensional sweep.

One simple way to sweep over this space is to align g(t) to arepresentative “phantom” user. Specifically, the base station internallygenerates a random fictitious channel gain vector h(t)=[h_(1(t) . . . h)_(k)(t)] according to the distribution function of a typical user in thesystem. For example, this vector can be generated by having Kcomponents, h_(k)(t), be independent and identically distributed lowpassGaussian random processes. The gain h(t) can be seen as the channel gainof a hypothetical user. The base station then sets the beamformingcoefficients g(t) to be aligned to this user. That is,g (t)= h (t)/∥ h (t)∥.As h(t) varies in time, the beamforming coefficients g(t) will sweepover the set of possible optimal beamforming coefficients. If theprobability distribution of channel gain h(t) matches the distributionfor the users, the beamforming coefficients g(t) will have correctdistribution to optimally visit each of the possible antennaconfigurations.

Any lowpass Gaussian random process can be used to generate thecomponents of h(t). The bandwidth of the process determines the rate ofvariation of g(t), and thereby provides an adjustable parameter tradingoff the sweep frequency with the required channel tracking bandwidth atthe users.

One simple method of extending a sweeping pattern for a single pipe tomultiple pipes is to offset the beamforming coefficients by fixedrotations. Specifically, we first determine the sweeping pattern forsome pipe, say pipe 1. Let g ¹ (t) denote the beamforming coefficientfor that pipe. g ₁(t) can be generated using the method discussed abovewith respect to g(t). The beamforming coefficients in the remainingpipes can then be set as some fixed rotation from g ₁(t). That is,g _(m)(t)=U _(m) g ₁(t), m=1, . . . , M,  (5)where U_(m)'s are a set of M constant unitary K×K matrices, and where mis the pipe index.

The matrices U_(m)'s should be selected so that, at any time t, the setof coefficients g _(m)(t)'s are “maximally” separated, insuring that,for any user at any time, the cannel condition of the best pipe issufficiently good. To define this criteria more precisely, let

${{G\left( {U_{1},\ldots\mspace{14mu},U_{M}} \right)} = {\underset{\_}{E}{\max\limits_{{m = 1},\ldots,M}{{h^{\prime}U_{m}g_{1}}}^{2}}}},$where the expectation is over h and g₁, which we assume to beindependent K-dimensional complex Gaussian random vectors. Given achannel gain h, the signal-to-noise ratio (SNR) on pipe m, isproportional to |h′g_(m)|²=|h′U_(m)g₁|². Therefore, the quantity Grepresents the expected SNR of the best pipe among the M pipes. One wayto select the U_(m)'s is to maximize this quantity, i.e.,

$U_{1},\ldots\mspace{14mu},{U_{M} = {\underset{U_{1},\ldots,U_{M}}{argmax}{{G\left( {U_{1},\ldots\mspace{14mu},U_{M}} \right)}.}}}$

The maximization problem is essentially equivalent to the problem offinding M vectors uniformly on the K-dimensional sphere. When K=2, theoptimal matrices are the rotation matrices,

${U_{m} = \begin{pmatrix}{\cos\;\theta_{m}} & {\sin\;\theta_{m}} \\{{- \sin}\;\theta_{m}} & {\cos\;\theta_{m}}\end{pmatrix}},\mspace{14mu}{\theta_{m} = \frac{\left( {m - 1} \right)\pi}{M}}$For higher dimensional K, procedures for finding good suboptimalmatrices are available.

Various features of the present invention are implemented using modules.Such modules may be implemented using software, hardware or acombination of software and hardware.

Many of the above described methods or method steps can be implementedusing machine executable instructions, such as software, included in amachine readable medium such as a memory device, e.g., RAM, floppy disk,etc. to control a machine, e.g., general purpose computer with orwithout additional hardware, to implement all or portions of the abovedescribed methods. Accordingly, among other things, the presentinvention is directed to a machine-readable medium including machineexecutable instructions for causing a machine, e.g., processor andassociated hardware, to perform one or more of the steps of theabove-described method(s).

Numerous additional variations on the methods and apparatus of thepresent invention described above will be apparent to those skilled inthe art in view of the above description of the invention. Suchvariations are to be considered within the scope of the invention. Themethods and apparatus of the present invention may be used with CDMA,orthogonal frequency division multiplexing (OFDM), or various othertypes of communications techniques which may be used to provide wirelesscommunications links between access nodes such as base stations andwireless terminals such as mobile nodes. Accordingly, in someembodiments base stations establish communications links with mobilenodes using OFDM or CDMA. In various embodiments the mobile nodes areimplemented as notebook computers, personal data assistants (PDAs), orother portable devices including receiver/transmitter circuits and logicand/or routines, for implementing the methods of the present invention.

What is claimed is:
 1. A method for providing channel diversity in amultiple access wireless communication system, the method comprising:transmitting a plurality of signals using a plurality of communicationchannels, wherein each of the plurality of communication channels has aphysical characteristic that is detectable by a wireless terminal andeach of the plurality of signals has a signal transmissioncharacteristic; changing at least one signal transmission characteristicto modify the physical characteristic of at least one of the pluralityof communication channels as received by the wireless terminal;receiving a feedback information from the wireless terminal wherein thefeedback information is based on the modified physical characteristic;and selecting one of the plurality of communication channels based onthe feedback information for use in transmission, wherein the changingat least one signal transmission characteristic is performedperiodically with a periodicity that is longer than or equal to aduration between receiving two feedback information.
 2. The method ofclaim 1, further comprising transmitting a plurality of pilot signalsusing the plurality of communication channels, wherein the plurality ofpilot signals is independent from the plurality of signals.
 3. Themethod of claim 1 wherein the physical characteristic being modifiedcomprises at least one of phase variation or amplitude variation.
 4. Themethod of claim 1 wherein the feedback information is a channel qualityinformation.
 5. The method of claim 4 wherein the channel qualityinformation includes a signal-to-noise ratio (SNR) or a signal-tointerference ratio (SIR).
 6. The method of claim 5 further comprising:receiving a channel quality report from the wireless terminal afterperforming the selecting step; and reselecting a different one of theplurality of communication channels for transmission based on thechannel quality report.
 7. The method of claim 1 wherein the at leastone signal transmission characteristic being changed comprises aparameter controlling the antenna pattern of the transmission.
 8. Themethod of claim 7 wherein the step of changing is performed on at leastthree signal transmission characteristics to modify the physicalcharacteristics of at least three of the plurality of communicationchannels as received by the wireless terminal, and is performed in asynchronized manner to maintain at least one difference in the physicalcharacteristics.
 9. The method of claim 8 wherein the at least onedifference in the physical characteristics is one of gain, phase oramplitude.
 10. A method for facilitating channel diversity in a multipleaccess wireless communication system, the method comprising: receiving aplurality of signals from a plurality of communication channels havingat least one physical characteristic modified by at least one signaltransmission characteristic of the plurality of signals; measuringchannel quality information based on the at least one physicalcharacteristic of the plurality of communication channels, wherein thechannel quality information comprises at least one of a signal-to-noiseratio (SNR) or a signal-to-interference ratio (SIR) relating to at leastone of the plurality of communication channels; and transmitting thechannel quality information to a base station, wherein the channelquality information is used to select one of the plurality ofcommunication channels for use in transmission, wherein the channelquality information is transmitted periodically with a periodicity thatis longer than or equal to a duration between receiving two feedbackinformation.
 11. An apparatus for providing channel diversity in amultiple access wireless communication system, the apparatus comprising:means for transmitting a plurality of signals using a plurality ofcommunication channels, wherein each of the plurality of communicationchannels has a physical characteristic that is detectable by a wirelessterminal and each of the plurality of signals has a signal transmissioncharacteristic; means for changing at least one signal transmissioncharacteristic to modify the physical characteristic of at least one ofthe plurality of communication channels as received by the wirelessterminal; means for receiving a feedback information from the wirelessterminal wherein the feedback information is based on the modifiedphysical characteristic; and means for selecting one of the plurality ofcommunication channels based on the feedback information for use intransmission, wherein the means for changing changes the at least onesignal transmission periodically with a periodicity that is longer thanor equal to a duration between receiving two feedback information. 12.The apparatus of claim 11, further comprising means for transmitting aplurality of pilot signals using the plurality of communicationchannels, wherein the plurality of pilot signals is independent from theplurality of signals.
 13. The apparatus of claim 11 wherein the physicalcharacteristic being modified comprises at least one of phase variationor amplitude variation.
 14. The apparatus of claim 11 wherein thefeedback information is a channel quality information.
 15. The apparatusof claim 14 wherein the channel quality information includes asignal-to-noise ratio (SNR) or a signal-to interference ratio (SIR). 16.The apparatus of claim 15 further comprising: means for receiving achannel quality report from the wireless terminal, wherein the means forreceiving is enabled after one of the plurality of communicationchannels is selected; and means for reselecting a different one of theplurality of communication channels for transmission based on thechannel quality report.
 17. The apparatus of claim 11 wherein the atleast one signal transmission characteristic being changed comprises aparameter controlling an antenna pattern of the transmission.
 18. Theapparatus of claim 17 wherein the means for changing changes at leastthree signal transmission characteristics to modify the physicalcharacteristics of at least three of the plurality of communicationchannels as received by the wireless terminal, and performs the changesin a synchronized manner to maintain at least one difference in thephysical characteristics.
 19. The apparatus of claim 18 wherein the atleast one difference in the physical characteristics is one of gain,phase or amplitude.
 20. An apparatus for facilitating channel diversityin a multiple access wireless communication system, the apparatuscomprising: means for receiving a plurality of signals from a pluralityof communication channels having at least one physical characteristicmodified by at least one signal transmission characteristic of theplurality of signals; means for measuring channel quality informationbased on the at least one physical characteristic of the plurality ofcommunication channels, wherein the channel quality informationcomprises at least one of a signal-to-noise ratio (SNR) or asignal-to-interference ratio (SIR) relating to at least one of theplurality of communication channels; and means for transmitting thechannel quality information to a base station, wherein the channelquality information is used to select one of the plurality ofcommunication channels for use in transmission, wherein the channelquality information is transmitted periodically with a periodicity thatis longer than or equal to a duration between receiving two feedbackinformation.
 21. A base station for providing channel diversity in amultiple access wireless communication system, the base stationcomprising: a transmitter for transmitting a plurality of signals usinga plurality of communication channels, wherein each of the plurality ofcommunication channels has a physical characteristic that is detectableby a wireless terminal and each of the plurality of signals has a signaltransmission characteristic; a processor coupled to the transmitter forchanging at least one signal transmission characteristic to modify thephysical characteristic of at least one of the plurality ofcommunication channels as received by the wireless terminal; and areceiver coupled to the processor for receiving feedback informationfrom the wireless terminal wherein the feedback information is based onthe modified physical characteristic; and wherein the processor selectsone of the plurality of communication channels based on the feedbackinformation for use in transmission, wherein the processor changes atleast one signal transmission characteristic periodically with aperiodicity that is longer than or equal to a duration between receivingtwo feedback information.
 22. The base station of claim 21, wherein thetransmitter transmits a plurality of pilot signals using the pluralityof communication channels, and the plurality of pilot signals isindependent from the plurality of signals.
 23. The base station of claim21 wherein the physical characteristic being modified comprises at leastone of phase variation or amplitude variation.
 24. The base station ofclaim 21 wherein the feedback information is a channel qualityinformation.
 25. The base station of claim 24 wherein the channelquality information includes a signal-to-noise ratio (SNR) or asignal-to interference ratio (SIR).
 26. The base station of claim 25wherein: the receiver receives a channel quality report from thewireless terminal after one of the plurality of communication channelsis selected; and the processor reselects a different one of theplurality of communication channels for transmission based on thechannel quality report.
 27. The base station of claim 21 wherein the atleast one signal transmission characteristic being changed comprises aparameter controlling an antenna pattern of the transmission.
 28. Thebase station of claim 27 wherein the processor changes at least threesignal transmission characteristics to modify the physicalcharacteristics of at least three of the plurality of communicationchannels as received by the wireless terminal, and performs the changesin a synchronized manner to maintain at least one difference in thephysical characteristics.
 29. The base station of claim 28 wherein theat least one difference in the physical characteristics is one of gain,phase or amplitude.
 30. A mobile terminal for facilitating channeldiversity in a multiple access wireless communication system, the mobileterminal comprising: a receiver for receiving a plurality of signalsfrom a plurality of communication channels having at least one physicalcharacteristic modified by at least one signal transmissioncharacteristic of the plurality of signals; a processor coupled to thereceiver for measuring channel quality information based on the at leastone physical characteristic of the plurality of communication channels,wherein the channel quality information comprises at least one of asignal-to-noise ratio (SNR) or a signal-to-interference ratio (SIR)relating to at least one of the plurality of communication channels; anda transmitter for transmitting the channel quality information to a basestation, wherein the channel quality information is used to select oneof the plurality of communication channels for use in transmission,wherein the channel quality information is transmitted periodically witha periodicity that is longer than or equal to a duration betweenreceiving two feedback information.
 31. A non-transitorycomputer-readable medium storing a computer program, wherein executionof the computer program is for: transmitting a plurality of signalsusing a plurality of communication channels, wherein each of theplurality of communication channels has a physical characteristic thatis detectable by a wireless terminal and each of the plurality ofsignals has a signal transmission characteristic; changing at least onesignal transmission characteristic to modify the physical characteristicof at least one of the plurality of communication channels as receivedby the wireless terminal; receiving a feedback information from thewireless terminal wherein the feedback information is based on themodified physical characteristic; and selecting one of the plurality ofcommunication channels based on the feedback information for use intransmission, wherein the changing at least one signal transmissioncharacteristic is performed periodically with a periodicity that islonger than or equal to a duration between receiving two feedbackinformation.
 32. A non-transitory computer-readable medium storing acomputer program, wherein execution of the computer program is for:receiving a plurality of signals from a plurality of communicationchannels having at least one physical characteristic modified by atleast one signal transmission characteristic of the plurality ofsignals; measuring channel quality information based on the at least onephysical characteristic of the plurality of communication channels,wherein the channel quality information comprises at least one of asignal-to-noise ratio (SNR) or a signal-to-interference ratio (SIR)relating to at least one of the plurality of communication channels; andtransmitting the channel quality information to a base station, whereinthe channel quality information is used to select one of the pluralityof communication channels for use in transmission, wherein the channelquality information is transmitted periodically with a periodicity thatis longer than or equal to a duration between receiving two feedbackinformation.